implemented dual Nextcloud push registration with separate general and talk apptypes to ensure reliable Talk notification delivery; introduced stacked MessagingStyle notifications for chat threads with support for circular conversation avatars and disk caching

lib/api/marianumconnect/queries/push_device_register/push_device_register.dart

@@ -19,6 +19,7 @@ class PushDeviceRegister {
     required String userPublicKey,
     required String pushToken,
     required String platform,
+    required String registrationType,
     String? appVersion,
   }) async {
     try {
@@ -30,6 +31,9 @@ class PushDeviceRegister {
           'userPublicKey': userPublicKey,
           'pushToken': pushToken,
           'platform': platform,
+          // 'general' | 'talk' — the backend derives the NC hash comparison
+          // value from it (general = sha512(token), talk = sha512(token+'#talk')).
+          'registrationType': registrationType,
           'appVersion': ?appVersion,
         },
       );